Conventional cellular mobile radiotelephone subscriber units (CMRs) are capable of communicating on at least one of two independent sets of frequencies. However, a conventional cellular base system communicates on only one of these two sets of frequencies. Thus, a cellular base system may be referred to as an "A" system if it uses one set of frequencies or a "B" system if it uses the other set of frequencies. This dual-system approach to radiotelephony results from regulations which are intended, at least in part, to promote competition in providing radiotelephone services. When A and B cellular systems are both located in a single geographic area, radiotelephone service customers may choose to subscribe to either the A or B system in accordance with which system provides the best value to the customer. B systems are often referred to as "wireline" carriers because B systems are typically operated by the companies that provide wireline telecommunication services where the B systems are located. A systems are often referred to as non-wireline carriers because they are operated by companies other than the companies that operate the competing B systems. These frequency allocations are not necessarily permanent.
Each frequency set in a given cellular service area is assigned to one and only one service company. However, in different service areas the same frequency set may be assigned to different service companies (much as a television channel may be assigned to an affiliate of one network in one city and to an affiliate of another network in another city).
The home frequency set of a given telephone unit is the set of frequencies which the unit will ordinarily attempt to use. It will depend in large part on which service company is subscribed to by the telephone unit's user: if the user is a subscriber of a nonwireline service company, the user's home frequency set will be the A frequencies, and vice versa.
As will be familiar to those of ordinary skill, a frequency set typically includes paging channels and associated signalling channels, as well as voice channels. The paging and signalling channels are used for preliminary coded communications between a cellular telephone and a cell site in setting up a telephone call, after which a voice channel is assigned for the telephone's use on that call.
Each service company broadcasts a unique System Identification Number (SID) on all paging channels of the frequency sets on which it provides service in a given service area. A suitably equipped cellular telephone can thus determine which service company is providing service on a given paging channel by identifying the SID. Usually the SID contains three digits.
Receipt of a home SID is not necessarily required to be able to place a call. Many service companies have reciprocal billing arrangements with one another, meaning that a call can be placed on a frequency associated with a non-home cellular system. However, use of a non-home service company in this manner to place a call may result in the imposition of a surcharge (e.g., a fixed surcharge or a higher per-unit rate).
Furthermore, if the non-home service company does not have a reciprocal billing arrangement with the user's home service company, as a practical matter the user may not be able to place a call at all. Even though the telephone unit is capable of establishing a connection via the carrier signal, the non-home service company's switching equipment typically will not allow the user to do anything with the connection without a way to bill the user for his or her usage. Some service companies automatically switch calls of this kind to an operator who can take down a credit card number. However, absent a billing arrangement of some kind, no call can be completed.
Cellular telephones typically include status indication displays such as status lights. The "roam" light indicates that the telephone has detected a non-home SID on a carrier signal and can make at least a connection with that company's service via the carrier signal. The "no-service" light indicates that no cellular connections are available.
When a customer subscribes to a cellular system (either an A or B system), that system becomes the subscriber/customer's home system. The company that operates a customer's home system collects billing information and bills the customer for the customer's use of the home system's radiotelephone services. Whenever a customer is operating his or her CMR on a system other than the customer's home system, the customer is engaging in an activity known as roaming. The cellular system upon which a roaming CMR is operating is viewed as a foreign system.
Customers may receive telecommunication services while roaming. However, the home system company and the foreign system company must cooperate with one another before roaming services are permitted. For example, the foreign system must transfer call record information to the home system, and the home system must bill for, collect funds for, and distribute funds back to the foreign system for the roaming telecommunication services. Accordingly, the customer must typically pay additional charges when the customer uses roaming services.
When the customer operates his or her CMR in the area covered by the CMR's home system, no serious cellular base system selection choice needs to be made. The customer will almost always want to use the home system because the charges for home-system telecommunication services will be less. On the other hand, when the customer is roaming away from the home system, a choice of whether to operate a CMR on an A or B foreign system must be made.
Conventional CMRs employ any one of several different programmable selection processes in choosing a cellular system upon which to operate. For example, a CMR may prefer an A system but accept a B system if an A system is not available. This is called an A/B selection process. Conversely, a CMR may prefer a B system but accept an A system if a B system is not available. This is called a B/A selection process. Alternatively, a CMR may select only A systems (A-Only) or only B systems (B-Only) regardless of whether a competing system is available. Furthermore, a CMR may select only the home system so that roaming is prohibited. Other selection processes may be implemented as well. A CMR will typically utilize a default selection process which is consistent with its home system. For example, if a CMR's home system is an A system, then the CMR will typically utilize the A-Only or A/B selection processes as a default selection process.
Conventional CMRs permit alteration of the default selection process. However, this feature is not often used by customers because it is difficult to accomplish through a CMR's handset and because an intelligent nondefault selection process setting requires an understanding of cellular radiotelephony that many customers do not possess. Accordingly, providers of cellular services recognize that, for the most part, customers do not alter the default process selection setting.
The default selection process setting tends to limit any benefits that competition between A and B systems may provide while roaming. While the default setting favors the home system, at the same time it favors approximately one half of the numerous potential foreign systems over the other half of potential foreign systems. Assuming that this default setting does not change, in providing radiotelephone services to roaming customers one of each foreign area's two competing cellular systems benefits from an equipment-caused bias. As a result, the favored foreign systems need not aggressively price roaming services, and they need not be exceptionally cooperative with a roamer's home system, because it is highly probable that a roamer will use the favored foreign system regardless of cost. Consequently, roaming costs to a customer remain undesirably high.
While conventional CMRs can be configured so that the default system selection process setting is easily alterable, such configurations do not solve the problems faced by roaming customers. A typical customer is not prepared to make, or interested in making, a purchasing decision with respect to telecommunication services every time he or she is roaming. While roaming, a customer is unlikely to know the rates charged by the competing foreign cellular systems. Moreover, the rate structures may be complicated, and they may change from time to time. Thus, when a customer is roaming, he or she typically does not possess sufficient information upon which to base an intelligent foreign cellular system selection decision, even if the CMR has the capacity for such a selection. Typically the subscriber does not desire to go to the time and trouble to obtain this information, and mark the necessary alterations in the operation of the CMR.